Within the field of digital communications, multilevel modulation is used to map a number of bit sequences to a signal alphabet comprising a number of signal symbols, i.e. a number of points in signal space. For example, a bit sequence may be mapped onto a point in a complex signal space. A signal alphabet of size M allows log2(M) bits to be mapped to each symbol. However, when symbols are received at a receiver, they may be affected by noise, thereby affecting the decoding of the signal when retrieving the transmitted bit sequence. If multilevel modulation is used in conjunction with channel coding, many channel decoders, such as iterative decoders based on the BCJR algorithm, require likelihood values for the received bits, so-called soft values, as an input. A soft value corresponds to a likelihood value of a single bit being 0 or 1.
Examples of multilevel modulation include multi-amplitude level modulation in Pulse Amplitude Modulation (PAM), multi signal point modulation in Quadrature Amplitude Modulation (QAM), or the like.
For example, an emerging technology for wideband digital radio communications of Internet, multimedia, video and other capacity-demanding applications in connection with the third generation of mobile telephone systems is the evolving Wideband Code Division Multiple Access (WCDMA) specified as part of the 3GPP standardisation organisation. Within this technology, High Speed Downlink Packet Access (HSDPA) is provided including a high speed downlink shared channel (HS-DSCH) which uses 16-QAM. In 16-QAM, M=16, i.e. each symbol in the signal alphabet represents 4 bits. Future releases may comprise even larger constellation sizes such as 64-QAM. Unlike QPSK, 16-QAM also includes amplitude information into the modulation.
However, in modulation schemes such as 16-QAM that include amplitude information in the modulation, it is a problem that the quality of the signal decoding is sensitive to amplitude distortions.